It is old to provide a hydraulically set and mechanically released well packer as disclosed in U.S. Pat. Nos. 3,456,723 and 3,603,388. The present invention is directed to an improved well packer which is simpler and less expensive than the prior art. The present packer is hydraulically set by dual opposing pistons actuated by hydraulic pressure, one of which sets the slips and the other which sets the packer seal in unison centralizing the packer and the well. The packer is released by the simple straight upward motion. A single one-way clutch is connected between the pistons for allowing simultaneously movement of one of the pistons upwardly and the other of the pistons downwardly for setting and holding the packer centrally in the conduit. Various improvements are provided in the packer to simplify its construction and operation.